The invention relates generally to the monitoring of articles.
More particularly, the invention relates to the monitoring of printed sheets or other printed products which are to undergo processing.
A known method of monitoring printed sheets compares a printed sheet which is to be passed through processing stations with a reference sheet. This method is mainly used in gathering machines having magazines for stacks of sheets or in other machines of similar function.
When a magazine in a gathering machine is filled, it can happen that the printed sheets are placed in the wrong magazine or are placed in the proper magazine in the wrong position. The German Offenlegungsschrift 38 06 125 overcomes this difficulty by conveying a sheet which is held in position on the feeding drum of the gathering machine past a conventional optoelectric sensor. After the first properly positioned sheet has been engaged by the feeding drum and the latter has been rotated through a predetermined angle, the brightness of a light spot on the sheet is measured within a scanning field of like color and the resulting luminosity stored as a reference value. The luminosities of the following sheets are likewise measured at the predetermined angle of rotation of the feeding drum and, when a deviation from the reference value occurs, a control pulse is generated to stop the gathering machine or produce a signal.
A sheet is considered to be properly positioned on the feeding drum as long as its position is within a certain tolerance range. Therefore, the scanning range must be as large as the tolerance range. If these ranges do not coincide, properly positioned sheets can also lead to so-called "stoppers".
It nevertheless remains possible to check a printed sheet for identity with a reference sheet even when the scanning range which can be established within the printed area of the reference sheet is smaller than the position tolerance range, and even when the position of the sheet to be checked deviates slightly from that of the reference sheet so that an error signal is generated. However, in the extreme, the known method then requires, aside from a determination of the color of a characteristic region of a printed sheet, a determination of the position of a single line along a single direction. In other words, this situation can require the detection of a single point or a single line. Since such detection is affected by the accuracy of the angle of rotation of the feeding drum relative to the position of the sensor, this method is troublesome and unreliable.